You can't walk in and pick a table. Period. At least not until the upcoming MDR changes are effective.
At best you can ask for a table in a certain section or with a specific wait team, or a 2- or 4- top, or a table by a window, perhaps. Just because you see a table empty (and even set) doesn't mean their's a wait team available to work it as soon you sit down.
Once the upcoming MDR changes are effective and there is assigned dining agan, you can probably get your assigned table changed, but only to another table that's available, not to any table of your choosing.